Neptune's Great Dark Spot

This is how Neptune's Great Dark Spot and rings may have looked in 1989 from a position just beneath Neptune's ring plane. The outermost Adams ring is near the top of the frame, and beneath that is the much broader and diffuse Lassell ring. Further in toward Neptune and abutting the Lassell ring is the thin LeVerrier ring, and beyond that is the diffuse Galle ring.

The Great Dark Spot is believed to be a storm similar to, but only half the size of, Jupiter's Great Red Spot. While Jupiter's Great Red Spot has been raging for at least 400 years, subsequent observations of Neptune's Great Dark Spot in 1994 by the Hubble Space Telescope revealed that this storm has since disappeared.

The Great Dark Spot was a very dynamic weather system, generating massive, white clouds similar to high-altitude cirrus clouds on Earth. Unlike cirrus clouds on Earth however, which are composed of crystals of water ice, Neptune's cirrus clouds are made up of crystals of frozen methane. Neptune's clouds are driven by winds of 1,200 mph, the fastest winds of any planet in the Solar System. How such high-velocity winds come to be on a planet so far from the Sun is
still a mystery.